Magnetic structure for traveling wave tubes



y 12, 1964 D. c. BUCK ETAL 3,133,226

MAGNETIC STRUCTURE FOR TRAVELING WAVE TUBES I Filed Oct. 7, 1960 2 Sheets-Sheet 1 Fig.l.

- INVENTORS WITNESSES. Daniel C. Bu ck und Em/-82 Curl H. Scullm.

ATT lglEY May 12, 1964 D. c. BUCK ETAL MAGNETIC STRUCTURE FOR TRAVELING WAVE TUBES Filed Oct. '7, 1960 2 Sheets-Sheet 2 Fig 6 United States PatentO 3,133,226 MAGNETIC STRUCTURE FOR TRAVELING WAVE TUBES Daniel C. Buck, Veteran Township, Chemung County, and (Iari H. Scullin, Horseheads Township, Chemung County, N.Y., assignors to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsyivania Fiied Oct. 7, 1960, Ser. No. 61,169 Claims. (Cl. 3153.5)

This invention relates to apparatus including magnetic structures, and more particularly to such apparatus including traveling wave tubes wherein an electron beam is focused by a magnetic field along the relatively long path.

In a traveling wave tube, an electron beam is projected through an interaction space, generally defined by a helix type structure. The electron beam is made to interact with an electromagnetic wave traveling along the helix structure. A signal to be amplified is introduced into the interaction space by being applied to the helix and amplification occurs due to the interaction of the electromagnetic fields of the signal and the electron beam. In order to attain proper operation of the traveling wave tube, a magnetic field is provided to restrain the electron beam in agenerally cylindrical form so that it does not impinge on the helix of the transmission circuit. In such a structure, it is necassary to provide a magnetic field uniformly distributed along the entire path of the electrons. It is therefore necessary that the magnetic field axis be aligned with the tube axis, so that the flux generated by the magnet does not cross the axis of the system. The general cause of the magnetic field deviating from an axial direction is non-homogenities of the magnetic structure and stray magnetic fields.

T hedemagnetization characteristic of a magnetic material cannot be accurately controlled in production to better than for the important parameters of flux density and demagnetization force. In certain permanent magnet structures utilized in traveling wave tubes, at re-entrant type assembly is utilized in which at least two bar magnets are utilized diametrically opposed on opposite sides of the tube axis. In such a structure, it is desirable that the diametrically opposed magnets have identical parameters. In this invention by providing a magnetic shield exterior to the permanent magnets one is able to compensate for the different properties of the permanent magnets and thereby realize a uniform axial magnetic field.

It is accordingly, an object of this invention to provide an improved magnetic focusing structure for charged particles in those devices utilizing a long charged particle path.

It is another object to provide an improved magnetic focusing structure to maintain a uniform axial magnetic field distribution within an elongated air gap.

It is a further object to provide an improved focusing structure to prevent impingement of electrons on the slow wave structure, input and output couplers, and other electrodes of a traveling wave tube. 1

A more general object of this invention is to provide an improved magnetic circuit for producing a uniform axial magnetic field in a charged particle beam apparatus.

These and other objects of our invention will be apparent from the following description taken in accordance with the accompanying drawings throughout which like reference characters indicate like parts, which drawings form a part of this application, and in which:

FIGURE 1 is a sectionalview of a magnetic assembly embodying our invention in conjunction with a traveling wave tube taken along the axis of the traveling wave tube;

"ice- FIGURE 2 shows a transverse sectional view of FIG- URE 1 taken along the line IIII;

FIGURE 3 is an enlarged sectional view of a portion of the shunt member;

FIGURE 4 illustrates the position of the shunt with respect to the magnetic structure;

FIGURE 5 illustrates the demagnitization and energy product curves of two permanent magnets; and

FIGURE 6 illustrates a magnetic field of a magnetic structure without compensation.

Referring now to FIGURES 1 and 2 of the drawing, there is shown one specific embodiment of this invention wherein a magnetic field is provided for focusing the electron stream in a traveling wave tube 10 by a permanent magnet assembly 12. The magnetic assembly 12 consists of two permanent bar magnets 14 and 16 positioned symmetrically with respect to the tube axis and diametrically opposite. In the specific illustration, the two magnet members 14 and 16 appear to be parts of a tubular magnet surrounding the tube 10. The permanent magnets 14 and 16 may take several different shapes determined primarily on the basis that they are able to accommodate transverse wave guides for the input and output radio frequency energy to the traveling wave tube 10. The magnets 14 and 16 have their magnetic axis symmetrically disposed about the tube axis and substantially parallel thereto. The magnets 14 and 16 are magnetized longitudinally with like poles adjacent to provide a magnetic field distribution along the tube axis. The magnets 14 and 16 may be of any suitable material such as alnico 5, an iron alloy containing aluminum, nickel, cobalt, copper and titanium.

Pole pieces 18 and 20, of suitable magnetic material such as soft iron, advantageously abut directly against the ends of the two magnet members 14 and 16 and include the space between the ends of the upper and lower permanent magnets 14 and 16. The pole pieces 18 and 20 are annular disc members positioned transverse to the tube axis. The pole pieces 18 and 2 3 are provided with centrally located apertures within which sleeve members 22 and 24 are positioned respectively within the apertures of pole pieces 13 and 20. The sleeve members 22 and 24 are of suitable soft magnetic material and in good magnetic contact with the pole pieces 18 and 20, respectively. The sleeve members 22 and 24 are coaxial with the tube 10. The internal dimensions of the sleeve members 22 and 24 are sufiicient to accommodate the tube 10.

The traveling wave tube 10 which may be of any suitable type known in the art is inserted into the space between the permanent magnets 14 and 16 and extends over the length of the assembly. The traveling wave tube 10 comprises an envelope of a suitable material such as glass including an enlarged portion 26 and a tubular portion 30. An electron gun assembly 28 is provided within the enlarged portion of the envelope and the tubular portion 30 of the envelope contains a slow wave circuit illustrated as helix transmission circuit 32 and a collector electrode 34. The electron gun 28 directs an electron beam through the helix 32 to the collector 34. The electron path thus defined is longitudinal along the tube axis.

Input and output waveguides 36 and 38 are positioned within the end portions of the magnet assembly transverse to the'axis of the traveling wave tube 10 and in energy coupled relation with the input and output ends of the piece 20 is positioned between the electron collector 34 and the output waveguide 38.

In order to provide an axial magnetic field, it is necessary to make the flux contributed by each of the permanent magnets 14 and 16 to the airgap equal.

FIGURE indicates two possible demagnetization curves 54 and 56 which represent the two permanent magnets 14 and 16. The curve 54 represents the magnet 14 and the curve 56 represents the magnet 16. It is obvious from the curve that the upper magnet 14 is contributing more flux to the gap than the lower magnet 16 and a flux pattern like that illustrated in FIG. 6 is the result. In order to compensate for the difference in magnetic properties of the two magnets, an adjustable magnetic shunt 60 is provided around the permanent magnets 14 and 16. By adjusting the eccentricity of the shunt 60 as illustrated in FIGURE 4, one is able to make the flux contributed by both of the magnets 14 and 16 to the useful gap equal. In the specific embodiment shown in FIGURES 1, 2 and 4, the shunt 60 consists of a cylindrical member surrounding the entire magnetic structure assembly. The wall 61 of the tubular shunt member consists of three layers of .0478 inch annealed soft magnetic sheet steel or annealed cold rolled steel and two layers 17 of .062 inch aluminum. The wall 61 is rolled onto brass rings 62 and the tubular wall member 61 is fastened to the brass rings 62 by brass screws 64. Two openings 65 diametrically opposite are provided in the wall 61 of the tubular member 60 for introduction of the waveguide members 36 and 38. The shunt member 60 is located with respect to the remaining portion of the magnetic assembly by screws 66 passing through the tubular shunt wall 61 and seated in grooves 68 provided in the periphery of the two pole pieces 18 and 20. In this manner by adjusting the three screw members 66, which are positioned at spaced points 120 apart about the periphery, one is able to shift the axis of the shunt 60 so that it is eccentric with respect to the mechanical axis of the two permanent magnet members 14 and 16 and as a result the magnetic axis of the magnetic assembly will be positioned on the tube axis.

In the specific embodiment shown, a generally tubular configuration of permanent magnets is shown and therefore a tubular shunt is utilized. This shunt 60, of course, also serves the function of providing shielding from stray magnetic fields. If the permanent magnets were of a bar type such that they were substantially parallel to each other, then it would be possible to utilize a plate shunt With respect to each magnet. It is also possible, if one so desires, to utilize a separate shielding member in addition to the shunt member.

It is also possible to use the external shunt to provide temperature compensation of the permanent magnets.

Although the present invention has been shown in only a few forms, it will be obvious to those skilled in the art that it is not so limited but is susceptible to various changes and modifications without departing from the spirit and scope thereof.

We claim as our invention:

1. In a device of the traveling wave tube type which includes means for producing an electron beam for flow along a given path and a radio frequency propagating structure disposed adjacent said path to enable interaction between the electrons of said beam and radio frequency energy propagated along said structure; an electron optical system disposed about said path along its length comprising a hollow cylindrical permanent magnet coaxial with respect to said path magnetized longitudinally in a manner to produce oppositely polarized poles substantially at the ends of said cylindrical magnet, and a'cylinder of saturable magnetic material disposed eccentrically about said magnet to orient the axis of the developed magnetic field with the path of said beam.

2. In a device of the traveling wave tube type which includes means for producing an electron beam for flow along a given path and a radio frequency propagating structure disposed adjacent said path to enable interaction 1 between the electrons of said beam and radio frequency energy propagated along said structure; a magnetic structure positioned about said path along its length for focusing and centering said beam Within said structure comprising at least two elongated permanent bar magnets magnetized longitudinally in a manner to produce oppositely polarized poles at the ends of said bar, said bar magnets positioned substantially parallel to and equally spaced from said electron path, and a member of soft magnetic material positioned exterior to said magnets with respect to said path to orient the axis of the magnetic field developed by said bar magnets with respect to said path.

3. In a device of the traveling wave tube type which includes an envelope having therein means for producing an electron beam for fiow along a given path, a radio frequency propagating structure disposed adjacent said path to enable interaction between the electrons of said beam and said radio frequency energy propagated along said structure, and an electron collecting means disposed at the opposite end of said propagating structure with respect to said beam producing means, a magnetic structure positioned about said envelope for focusing and centering said electron beam within said propagating structure comprising a cylindrical core of magnetic material concentric with respect to the axis of said beam and extending along the length thereof, end pole pieces at the ends of said core and transverse to said axis, said envelope positioned within openings provided in said pole pieces, magnetic shunt means surrounding said cylindrical core having pins extending radially inward and contacting said pole pieces to thereby displace said shunt eccentrically in a transverse direction to position the magnetic field axis of said core in registration with said electron beam path.

4. In a device of the traveling wave tube type which includes means for producing an electron beam for fiow along a given path and a radio frequency propagating structure disposed adjacent said path to enable interaction between the electrons of said beam and said radio frequency energy propagated along said structure; an electron optical system disposed about said path along its length comprising a pair of bar permanent magnets magnetized longitudinally in a manner to produce oppositely polarized poles substantially at the ends of each of said magnets, said bar magnets positioned parallel to and equally spaced from said electron path, and a plate of saturable magnetic material disposed exteriorly of said magnets and unequally spaced from said electron path to orient the axis of the magentic field produced by said magnets with the path of said beam.

5. In a device of the traveling wave tube type which includes means for producing an electron beam for flow along a given path and a radio frequency propagating structure disposed adjacent said path to enable interaction between the electrons of said beam and said radio frequency energy propagated along said structure; a magnetic structure positioned about said path along its length for focusing and centering said beam within said structure comprising an elongated permanent magnet member magnetized longitudinally in a manner to produce oppositely polarized poles at the ends of said member, said member positioned substantially parallel to said electron path, and a member of magnetic material positioned exterior to said magnet with respect to said path to orient the axis of the magnetic field produced by said magnet with respect to said path.

References Cited in the file of this patent UNITED STATES PATENTS 2,586,559 Page Feb. 9, 1952 2,701,321 Rich Feb. 1, 1955 2,828,434 Klein et al Mar. 25, 1958 2,871,395 Cioffi Jan. 27, 1959 

3. IN A DEVICE OF THE TRAVELING WAVE TUBE TYPE WHICH INCLUDES AN ENVELOPE HAVING THEREIN MEANS FOR PRODUCING AN ELECTRON BEAM FOR FLOW ALONG A GIVEN PATH, A RADIO FREQUENCY PROPAGATING STRUCTURE DISPOSED ADJACENT SAID PATH TO ENABLE INTERACTION BETWEEN THE ELECTRONS OF SAID BEAM AND SAID RADIO FREQUENCY ENERGY PROPAGATED ALONG SAID STRUCTURE, AND AN ELECTRON COLLECTING MEANS DISPOSED AT THE OPPOSITE END OF SAID PROPAGATING STRUCTURE WITH RESPECT TO SAID BEAM PRODUCING MEANS, A MAGNETIC STRUCTURE POSITIONED ABOUT SAID ENVELOPE FOR FOCUSING AND CENTERING SAID ELECTRON BEAM WITHIN SAID PROPAGATING STRUCTURE COMPRISING A CYLINDRICAL CORE OF MAGNETIC MATERIAL CONCENTRIC WITH RESPECT TO THE AXIS OF SAID BEAM AND EXTENDING ALONG THE LENGTH THEREOF, END POLE PIECES AT THE ENDS OF SAID CORE AND TRANSVERSE TO SAID AXIS, SAID ENVELOPE POSITIONED WITHIN OPENINGS PROVIDED IN SAID POLE PIECES, MAGNETIC SHUNT MEANS SURROUNDING SAID CYLINDRICAL CORE HAVING PINS EXTENDING RADIALLY INWARD AND CONTACTING SAID POLE PIECES TO THEREBY DISPLACE SAID SHUNT ECCENTRICALLY IN A TRANSVERSE DIRECTION TO POSITION THE MAGNETIC FIELD AXIS OF SAID CORE IN REGISTRATION WITH SAID ELECTRON BEAM PATH. 